1. Field of the Invention
This invention relates to sensors that are capable of determining the rate of flow of a fluid. More particularly, the sensors can be used to determine the concentration of an analyte in a sample of fluid in a continuous manner where the rate of flow of the sample of fluid is expected to vary during the determination.
2. Discussion of the Art
Determination of the rate of flow of a fluid is important in systems designed for chemical analysis of samples of fluid and delivery of chemicals such as drugs. When the rate of flow of a fluid is known, one can accurately determine the concentration of an analyte in the fluid, even if it is a slow-moving fluid, or the concentration of a chemical in a formulation being delivered from a reservoir.
Certain sensors, known as continuous monitoring biosensors, can be used to continuously monitor the concentration of an analyte in a biological sample under either in vivo or in vitro conditions. For example, the concentration of glucose in a body fluid can be monitored continuously so that the patient will have a better understanding of variations in concentration of glucose in the blood, and, consequently will be able to modify his diet or medications. In general, the concentration of glucose in the blood is expected to change significantly during normal activity, such as consuming a meal, exercising, etc. Biosensors that have been disclosed for this function are described in U.S. Pat. Nos. 4,953,552; 4,960,467; 5,243,982; 5,291,887; 5,362,307; 5,469,846; 5,507,288; 5,527,288; 5,636,632; 6,134,461, all of which are incorporated herein by reference. Methods and devices for extracting the biological sample from the body are described in U.S. Pat. Nos. 5,582,184; 5,820,570, 5,951,493; 5,885,211, 5,640,954, all of which are incorporated herein by reference.
Continuous monitoring biosensors are also used in pharmaceutical or industrial applications to monitor analytes in flow injection analysis (FIA) systems. In these applications, a pump, such as an automated syringe pump, can accurately control the rate of flow of the sample through the biosensor. Biosensors that have been disclosed for this function are described in U.S. Pat. No. 6,214,206, incorporated herein by reference. Biosensors that are suitable for use in applications involving continuous monitoring typically rely on either detection of at least one reaction product, such as hydrogen peroxide, or detection of electric current with the aid of a mediator. Such biosensors typically include two or three electrodes. A biosensor having three electrodes has a working electrode, a reference electrode, and a counter electrode. A biosensor having two electrodes has a working electrode and a dual-purpose electrode, which functions as both a reference electrode and a counter electrode.
When a biological sample is continuously drawn out of the body of a patient for continuous measurement of the concentration of an analyte, such as, for example, glucose, the rate of flow of the sample may vary on account of one or more reasons. For example, when the biological sample is interstitial fluid, the rate of excretion of interstitial fluid from the body can vary depending on the level of vacuum aiding in removal of the sample from the body of the patient or the hydration state of the patient. The level of vacuum can vary on account of a leak in the vacuum. When the patient is overhydrated, the rate of flow of interstitial fluid can be relatively high; when the patient is dehydrated, the rate of flow of interstitial fluid can be relatively low. Furthermore, healing or inflammation at the site of the body at which the interstitial fluid is collected during the course of operation of the biosensor can also result in variations in the rate of flow. Because the rate of diffusion of the analyte to the surface of the working electrode of the biosensor is dependent on rate of flow of the fluid emerging from the body, the response of the biosensor will be dependent upon the rate of flow of the fluid. Situations involving continuous removal of the biological sample from the body for continuous measurement of an analyte can be contrasted with situations where the biosensor is implanted in the body of a patient, wherein it is presumed that there are no variations in the rate of flow, because the biosensor is immersed in a pool of the sample. In pharmaceutical or industrial applications, the rate of flow is controlled externally, typically by means of a peristaltic pump or a motorized syringe pump.
In order to measure the rate of flow of a biological fluid as it is being drawn out of the body of a patient, an additional device can be used for the measurement of rate of flow of the sample. This additional device would have to be placed in the flow path of the sample, thereby causing an increase in the dimensions of the flow path, and hence, an increase the in the volume of sample required. Moreover, an additional measurement system would be required, whereby the cost of the biosensor system would be increased.
Other means for testing the rate of flow of fluid involve the introduction of air bubbles (i.e., an air plug) to the flow path. The introduction of air bubbles allows one to monitor the movement of the air-liquid interface. There are methods and devices available for the detection of air-liquid interface by thermal or spectroscopic methods, whereby the rate of flow of the fluid can be determined by determining the rate of flow of the air bubbles or the air-liquid interface.
A biosensor for continuously monitoring the concentration of an analyte, such as, for example, glucose, can be constructed by immobilizing an enzyme, such as, for example, glucose oxidase, on the surface of the working electrode of the biosensor. The analyte in the biological sample is oxidized at the working electrode and the electrochemically active reaction product, such as, for example, hydrogen peroxide, can diffuse to the surface of the working electrode and to the channel of the biosensor through which the biological sample is flowing, i.e., the flow channel. A portion of this electrochemically active reaction product can diffuse back to the working electrode from the flow channel, thereby affecting the response of the biosensor. At low rates of flow of biological samples, the extent of back diffusion is significantly higher than that at high rates of flow. As a result, the response of the biosensor, e.g., the current measured, is dependent upon the rate of flow of the sample, with the response being higher at lower rates of flow. For biosensors for continuous monitoring of biological fluid, particularly when the sample is drawn out of the body from an artificial opening in the skin, the rate of flow of the sample becomes extremely important.
Accordingly, it would be desirable to develop a biosensor for continuously monitoring a biological sample, without the need for any additional devices or additional steps to measure the rate of flow of the sample.
In one aspect, this invention provides various types of sensors that are capable of measuring the rate of flow of a fluid that passes over the electrodes of the sensor. In any of these sensors, an electrode, designated the flow rate-determining electrode, is used in conjunction with the conventional electrodes, e.g., the working electrode, the reference electrode, and the counter electrode, to determine the rate of flow of the fluid.
In another aspect, this invention provides a sensor for measuring the concentration of an analyte in a sample of fluid when the sample flows continuously over the electrodes of the sensor, especially when the rate of flow of the biological sample is relatively low. In another aspect, this invention provides a method for measuring the concentration of an analyte in a sample of fluid, wherein the rate of flow of the sample varies during the period of time that the sensor is in place. Sensors that are used for measuring one or more parameters of a biological sample, e.g., the concentration of an analyte, are referred to herein as biosensors.
In a preferred embodiment, the sensor employs four electrodes, namely, a working electrode, a reference electrode, a counter electrode, and a flow rate-determining electrode. Alternatively, a single electrode that performs both the function of the reference electrode and the function of the counter electrode can replace the reference electrode and the counter electrode. In addition, a dummy electrode or a blank electrode can be used to compensate for interference from electrochemically active species. The reagent(s) specific to the analyte of interest is (are) required to be deposited on the working electrode.
In addition to the electrodes normally contained in a sensor, e.g., the working electrode, the reference electrode, the counter electrode, the sensor comprises a flow rate-determining electrode, which measures the quantity of electrochemically active species that diffuses to the flow path of the fluid that flows in the flow channel of the sensor. A calibration relationship can be established for relating the rate of flow of the fluid and the response of the flow rate-determining electrode or for relating the rate of flow of the fluid and the ratio of the responses of the flow rate-determining electrode and the working electrode.
In one embodiment, the rate of flow of a sample of fluid can be determined from the response of the working electrode and the response of the flow rate-determining electrode. In a second embodiment, the rate of flow of a sample of fluid can be determined from the response of a working electrode that has been modified to render it insensitive to changes in the rate of flow of the sample and the response of a flow rate-determining electrode that has been designed to render it sensitive to changes in the rate of flow of the sample. In a third embodiment, the rate of flow of a sample of fluid can be determined from the response of the working electrode and the response of the flow rate-determining electrode along with the knowledge of the direction of flow of the sample.
The inclusion of the flow-rate determining electrode in the sensor enables the accurate determination of the concentration of the analyte regardless of the rate of flow of the sample. The flow rate-determining electrode is preferably fabricated at the same time as the other electrodes; no additional fabrication step is required.